In the semiconductor manufacturing industry, to extend resolution capabilities beyond those obtained with standard resist patterning techniques, various processes for pattern shrink have been proposed. These processes involve increasing the effective thickness of the resist pattern sidewalls to reduce (i.e., “shrink”) the spacing, for example, between adjacent lines or within a trench or hole pattern. In this way, features such as trenches and contact holes formed from the patterns can be made smaller. Known shrink techniques include, for example, chemical vapor deposition (CVD) assist, acid diffusion resist growth, thermal flow and polymer blend self-assembly.
The CVD assist shrink process (see K. Oyama et al, “The enhanced photoresist shrink process technique toward 22 nm node”, Proc. SPIE 7972, Advances in Resist Materials and Processing Technology XXVIII, 79722Q (2011)), uses a CVD-deposited layer formed over a photoresist pattern including, for example, contact hole, line/space or trench patterns. The CVD material is etched back, leaving the material on sidewalls of the resist pattern. This increases the effective lateral dimensions of the resist pattern, thereby reducing the open areas that expose the underlying layer to be etched. The CVD assist shrink technique requires use of CVD and etching tools which are costly, add to the complexity of the process and are disadvantageous in terms of process throughput.
In the acid diffusion resist growth process, also referred to as the RELACS process (see L. Peters, “Resists Join the Sub-λ Revolution”, Semiconductor International, 1999. 9), an acid-catalyzed crosslinkable material is coated over a positive tone developed (PTD) resist patterned surface. Crosslinking of the material is catalyzed by an acid component present in the resist pattern that diffuses into the crosslinkable material during a baking step. The crosslinking takes place in the material in the vicinity of the resist pattern in the acid diffusion region to form a coating on sidewalls of the pattern, thereby reducing the lateral dimension of open areas of the pattern. This process typically suffers from iso-dense bias (IDB), wherein growth of the crosslinked layer on the resist pattern occurs non-uniformly across the die surface depending on density (spacing between) adjacent resist patterns. As a result, the extent of “shrink” for identical features can vary across die based on pattern density. This can lead to patterning defects and variations in electrical characteristics across the die for what are intended to be identical devices.
Polymer blend self-assembly (see Y. Namie et al, “Polymer blends for directed self-assembly”, Proc. SPIE 8680, Alternative Lithographic Technologies V, 86801M (2013)) involves coating a composition containing an immiscible blend of hydrophilic and hydrophobic polymers over the photoresist pattern. The composition is then annealed, causing the polymers to phase separate, wherein the hydrophilic polymer preferentially segregates to the resist pattern sidewalls and the hydrophobic polymer fills the remainder of the volume between the resist pattern sidewalls. The hydrophobic polymer is next removed by solvent development, leaving the hydrophilic polymer on the resist pattern sidewalls. Polymer blend self-assembly has been found to suffer from proximity and size effects. As the shrink ratio is determined by the volume ratio of the two polymers, all features shrink by the same relative percentage rather than by the same absolute amount. This can lead to the same problems described with respect to the acid diffusion resist growth technique.
A polymer grafting shrink technique has also been proposed (see, e.g., U.S. Patent Application Pub. No. 2015/0086929A1). As shown in FIGS. 1A and 1B, in this process, a photoresist pattern 1 and substrate 2 is overcoated with a shrink composition 3 containing a polymer having a group which bonds to the surface of the resist pattern. Following rinse of residual unbound polymer with a solvent, a layer 3 of the bonded polymer from the shrink composition remains over the photoresist pattern. The inventors have observed that attachment of the polymer to the resist pattern can result in a footing layer 4 being formed on the substrate surface. The occurrence of footing can result from bonding of the polymer to the substrate surface and wetting of the polymer bonded to resist sidewalls onto the polymer bonded to the substrate. The occurrence of footing is undesirable in that it can result in patterning defects, for example, bridging defects or missing contact holes, which can adversely impact device yield.
There is a continuing need in the art for improved pattern treatment methods which address one or more problems associated with the state of the art and which allow for the formation of fine patterns in electronic device fabrication.